Created a scoped db_session with cleanup

[ankitchiplunkar]

Types of changes

What types of changes does your code introduce to ether_sql? Put an x in the boxes that apply

• [x] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Explain your changes

Does this close any currently open issues? https://github.com/analyseether/ether_sql/issues/38

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• [x] I have read the CONTRIBUTING doc
• [x] Lint and unit tests pass locally with my changes
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)

Where has this been tested?

Operating System: Ubuntu 16.04

Node: Parity and Infura